Extensive air showers, initiated in the Earth’s atmosphere by high-energy cosmic rays, emit pulsed radio signals with a typical duration of tens of nanoseconds. With the Auger Engineering Radio Array (AERA), consisting of 150 detector stations on an area of 17 km2, we measure these signals in the 30 to 80 MHz band. For air showers arriving within 60° from the vertical, the radio emission illuminates only limited areas on the ground, of order a few hundred meters in diameter. Consequently, radio signals are typically detectable only in a handful of AERA radio detector stations for a given air shower.

It has been long-expected that air showers arriving more horizontally, with angles of more than 60° from the vertical, should illuminate much larger ground areas with measurable radio signals. This is because for these geometries, the emission source will be much further away, and thus the emission – beamed in a narrow cone – is distributed over much larger areas.

A recent analysis by the Pierre Auger Collaboration has now proven that these expectations were indeed correct. Over the course of two years, and with an earlier (smaller) configuration of AERA, more than 500 air showers with zenith angles between 60 and 84° could be observed in coincidence between the Auger surface detectors and the AERA radio antennas. These air showers have been measured by up to 80 radio antennas at the same time, and up to distances from the air-shower axis of 2000 m or more, as is illustrated in Figure 1. In the ground-plane, such air showers illuminate areas of more than 100 km2 – much larger than AERA – with measurable radio signals.

Figure 1: This air shower arriving under an angle of 82.8° from the vertical (i.e., 6.2° above the horizon) was successfully measured with 74 AERA antenna stations, represented by the colored crosses in the left panel. Consequently, the radio-signal strength dependence on the lateral distance from the shower axis was measured with great detail. Measurable signals were detected up to more than 2000 meters from the axis, as shown in the right panel.

In fact, the size of the radio-emission footprint could be shown to increase with the inclination angle of the air shower, as is shown in Figure 2, in line with the expectation for beamed emission from a receding source which is not undergoing relevant scattering or absorption during atmospheric propagation.

Figure 2: The maximal distance from the shower axis at which a measurable radio signal is detected increases with the shower inclination as measured from the zenith.

In our analysis, we have also demonstrated that the measured radio-signal strengths are in agreement with state-of-the-art simulations, that the clear maximum visible in Figure 1 (right panel) is consistent with a “Cherenkov ring” expected because of the refractive index of the atmosphere, and that in individual events the “radio footprint” can even be considerably larger than the “particle footprint”.

Our findings open the door to measurements of extensive air showers on very large areas, and thus up to the highest cosmic-ray energies, with very sparse radio-antenna arrays. Indeed, we have demonstrated that air showers such as the one shown in Figure 1 can be measured with a radio-detector array on a 1.5 km hexagonal grid as is used by the Pierre Auger surface detector stations, as shown in Figure 3.

Figure 3: The same air shower as shown in Figure 1 as it would be seen with a thinned-out radio-detector array on a 1.5 km hexagonal grid.